Compression of gaseous streams containing carbon monoxide



March 4, 1969 D. D. MEHTA ET AL COMPRESSION OF GASEOUS STREAMSCONTAINING CARBON MONOXIDE Filed July 10, 1967 DINSHAW D. MEHTA AXELCHRISTENSEN I N VENTORS.

United States Patent 3,430,451 COMPRESSION 0F GASEOUS STREAMS CON-TAINING CARBON MONOXIDE Dinshaw D. Mehta, New York, N.Y., and AxelChristensen, Stamford, Conn., assignors to Chemical ConstructionCorporation, New York, N.Y., a corporation of Delaware Filed July 10,1967, Ser. No. 652,182 US. Cl. 62-85 Int. Cl. FZSb 47/00 16 ClaimsABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of theinvention The invention pertains to the compression of gaseous streamsconsisting of or containing carbon monoxide, such as methanol synthesisgas, which are to be compressed in centrifugal compressors which areconstructed or fabricated of structural elements consisting of carbonsteel as a material of construction. Such structural elements mayinclude the compressor casing, rotor or wheels.

Description 0 f the prior art Compression of methanol synthesis gas,which contains carbon monoxide, has been carried out in the prior art bythe use of piston-type reciprocating compressors. With the advent oflarge-scale plants which produce large tonnages of methanol per streamday, it has proven necessary for optimum economy to provide centrifugalcompressors for the compression of the methanol synthesis gas tosynthesis pressures of about 350 kg./sq. cm. These centrifugalcompressors have been provided with internal elements such as rotorblades which are fabricated of stainless steel, which resists attack bycarbon monoxide and does not form carbonyl compounds by reaction withcarbon monoxide in service. However, the use of stainless steel is acostly expedient and also introduces fabrication problems notencountered with carbon steel elements SUMMARY OF THE INVENTION Thepresent invention relates to the compression of gas streams whichconsist of or contain carbon monoxide. The invention is particularlydirected to the compression of methanol synthesis gas prior to highpressure methanol synthesis. The typical methanol synthesis gasprincipally contains carbon monoxide, carbon dioxide and hydrogen insuitable proportions for catalytic synthesis of methanol by reaction ofthe hydrogen with the carbon oxides.

In the present invention, a gaseous stream containing carbon monoxide,such as methanol synthesis gas, is compressed in a rotating centrifugalcompressor which has at least one carbon steel structural element. Thus,the compressor casing, rotor, wheels or blades or all of these elementsmay be fabricated of carbon steel. Within the context of the presentinvention, the term centrifugal compressor will be understood to includevarious types of rotating compressors such as conventional centrifugalcompressors, axial compressors etc., and the improvement of the presentinvention does not relate to prior art piston-type reciprocatingcompressors. In accordance with the present invention as relating tocentrifugal compressors, an improved method of preventing the reactionof carbon monoxide with carbon steel structural elements of thecompressor during compression is provided. The reaction of carbonmonoxide with carbon steel leads to carbonyl formation, with consequentcorrosion and weakening of the structural element, and entrainment ofiron carbonyl in the process stream. This reaction is prevented byinjecting a liquid oil such as hydrocarbon oil, mineral oil or siliconeoil into the feed gas stream being passed to the rotating centrifugalcompressor. The liquid oil must have a flash point above the maximumtemperature developed in the gas stream during compression, in order toavoid decomposition of the oil or reaction of the oil with the gasstream. The injected oil provides a thin film or coating on the internalstructural elements of the compressor, which effectively prevents thereaction of carbon monoxide in the gas stream with the carbon steelstructural elements of the compressor. The resulting compressed gasstream discharged from the centrifugal compressor is passed to aconventional gas-liquid separator, for separation of liquid oil from thecompressed gas stream. In most instances, the compressed gas stream willbe cooled prior to separation of the liquid oil component.

The principal advantage of the present invention is that the reaction ofthe carbon monoxide component of the gaseous stream with carbon steelstructural elements of the centrifugal compressor is effectivelyprevented. Thus, centrifugal compressors having carbon steel casings orother structural elements fabricated of carbon steel may be employed inthe compression of gaseous streams containing carbon monoxide, and theprior art requirement of stainless steel elements for this service hasnow been obviated. The fabrication or machining of carbon steel elementsis considerably simpler and less costly than the production of theseelements when stainless steel is the material of construcion. Inaddition, due to the injection of the liquid oil into the feed gasstream to the centrifugal compressor, the resultant coating of thecompressor internals results in the in situ lubrication of rotatingelements and support, which provides lenghened service life for theunit, and the oil coating on the centrifugal compressor internals suchas the rotating wheels and blades effectively prevents erosion andcavitation effects. Finally, an important advan age is that theentrainment or carryover of iron carbonyl into the process gas streambeing compressed is effectively prevented. This is highly important insuch instances as the compression of methanol synthesis gas, since thecarryover or deposition of iron carbonyl into the methanol synthesiscatalyst beds is effectively pre vented. At the operating temperaturesof the methanol catalyst beds, generally in the range of 330 C. to 400C., iron carbonyl will decompose in o iron and carbon monoxide. Thepresence of iron in the catalyst bed will promote a methanationreaction, which is most difficult to control and which leads to acondition generally known as a runaway reaction, which has beenresponsible for severe mechanical damages to catalyst baskets of severalmethanol plants.

It is an object of the present invention to provide an improved methodfor compression of gaseous streams containing carbon monoxide incentrifugal compressors.

Another object is to prevent carbonyl formation during the compressionof gaseous streams containing carbon monoxide in centrifugal compressorswhich are fabricated wi.h a carbon steel structural element.

A further object is to provide an improved method for the compression ofmethanol synthesis gas in centrifugal compressors which have at leastone carbon steel structural element in contact with the methanolsynthesis gas stream.

An additional object is to prevent the reaction of carbon monoxide withcarbon steel during the compression of a gaseous stream containingcarbon monoxide in a centrifugal compressor having an internal elementfabricated of carbon steel.

Still another object is to prevent the carryover of iron carbonyl intothe methanol synthesis catalyst bed, in instances when methanolsynthesis gas is compressed in centrifugal compressors having at leastone carbon steel structural element.

These and other objects and advantages of the present invention willbecome evident from the description which follows.

DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS Referring now tothe drawing, a flowsheet of a preferred embodiment of the invention ispresented. Stream 1 consists of a gaseous stream containing carbonmonoxide, and in this embodiment of the invention, stream 1 consists ofmethanol synthesis gas principally containing carbon monoxide, carbondioxide and hydrogen. Stream 2 consisting of a liquid oil such as ahydrocarbon oil, mineral oil or silicone oil is injected into stream 1.The oil stream 2 will have a flash point above 230 C., in order to avoidsubsequent oil decomposition or reaction with the synthesis gas streamduring compression. The resulting mixed gas-liquid stream 3 is passedinto the inlet port or wheel of rotating centrifugal compressor 4, andthe gas stream is compressed in unit 4. The liquid oil component ofstream 3 serves to coat the carbon steel internals of unit 4 with a thinfilm or coating of liquid oil, which effectively prevents the reactionof the carbon monoxide component of the gas stream with the carbon steelinternals or casing of unit 4, and thus prevents carbonyl formation.Unit 4 is driven by drive shaft 5, which extends to steam turbine 6 inthis embodiment of the invention. The steam turbine 6 provides themotive power for operation of the centrifugal compressors such as unit4. High pressure steam stream 7 is expanded through the blades or wheelsof unit 6 in accordance with conventional steam turbine practice, andlow pressure steam is removed from unit 6 via stream 8,

which may be passed to a vacuum or surface condenser.

Alternative drive units such as an electric motor or combustion gasturbine may be employed instead of or in addition to unit 6.

The compressed synthesis gas together with entrained liquid oil isdischarged from unit 4 via stream 9 at an elevated temperature generallyin the range of 130 C. to 200 C. Stream 9 is cooled in heat exchanger10, usually by heat exchange with boiler feed water or cooling water orboth, and is discharged via stream 11 at a reduced temperature generallyin the range of 30 C. to 80 C. Stream 11 now contains entrained waterdroplets produced by the condensation of water vapor from the gas phaseat elevated pressure and reduced temperature, as well as entrainedliquid oil. Although stream 11 may be passed directly to furthercompression, it will usually be preferable to remove the entrainedliquid phase from stream 11, and add a second stream of fresh liquid oilto the resulting gas stream. Stream 1] is therefore passed intogas-liquid separator 12, which is a conventional baffled or cyclonicunit for the separation of gas and liquid phases. A liquid phase stream13 containing water and oil is removed from separator 12. In mostinstances, stream 13 will be passed to an oil separator, not shown,where the oil and water phases separate from each other. The lower waterphase is discharged to a sewer or other disposal, while the oil phasemay be reprocessed to remove sludge etc. and then recycled.

A liquid-free intermediate pressure synthesis gas stream 14 is withdrawnfrom unit 12, and is preferably combined with a second liquid oil stream23, which is similar or identical in composition to stream 2 describedsupra. The resulting mixed gas-liquid stream 24 is passed into the inletport or wheel of rotating centrifugal compressor 15, and the gas streamis further compressed in unit 15. The liquid oil component of stream 24serves to coat the carlton steel internals of unit 15 with a thin filmor coating of liquid oil, which effectively prevents the reaction of thecarbon monoxide component of the gas stream with the carbon steelinternals or casing of unit 15, and thus prevents carbonyl formation.Unit 15 is driven by drive shaft 16, which is connected with drive shaft5 in this embodimen. of the invention, and rotates together with shaft5. Al ernatively, shaft 16 may be driven by a separate power source,such as an additional steam turbine, combustion gas turbine or electricmotor.

The final fully compressed synthesis gas together with entrained liquidoil is discharged from unit 15 via stream 17 at an elevated temperaturegenerally in the range of 130 C. to 200 C. Stream 17 is cooled in heatexchanger 18, usually by heat exchange with boiler feed water or coolingwater or both, and is discharged via stream 19 at a reduced temperaturegenerally in the range of 30 C. to C. An entrained liquid phaseprincipally consisting of water and oil is removed from stream 19, bypassing s'ream 19 into gas-liquid separator 20, which is similar inconfiguration and function to unit 12 described supra. A liquid phasestream 21 containing water and oil is removed from unit 20, and willusually be combined with stream 13 for subsequent processing asdescribed supra. The liquidfree high pressure synthesis gas stream 22 iswithdrawn from unit 20 and passed to methanol synthesis.

Numerous alternatives within the scope of the present invention willoccur to those skilled in the art. As mentioned supra, unit 6 mayalternatively be replaced by functionally equivalent drive units such asan electric motor or combustion gas turbine, or a plurality of units maybe employed in tandem. Stream 1 may consist of any gas stream consistingof or containing carbon monoxide, such as synthesis gas during variousstages of gas reforming, methanol synthesis gas, Fischer-Tropschsynthesis gas etc. As mentioned supra, in some instances stream 11 maybe directly passed into unit 15 and unit 12 and stream 23 may beomitted. Alternatively, only a partial separation of the liquid phasemay take place in unit 12, in which case stream 14 may be passeddirectly into unit 15, with the omission of stream 23. In someinstances, stream 9 may be at desired pressure for subsequent processingand unit 15 may be omitted. In this case,

stream 9 may be treated for the removal of entrainedv liq-uid oil, bypassing to a separation unit such as unit 12, or unit 10 may be providedfor cooling of the gaseous stream prior to passing to unit 12. More thantwo compression units such as units 4 and 15 may be provided in seriesor parallel or both. in which case liquid oil will usually be injectedinto the gas stream prior to each unit. In some instances of multi-stageseries compression, the temperature rise in the later or final stagesmay be small, and the intermediate gas stream temperature may besulficiently low, for example a temperature below C., that carbonmonoxide reaction and carbonyl formation will not occur. In this case,oil injection in the later stage or stages of compression may beomitted.

An example of an industrial application of the method of the presentinvention to the compression of methanol synthesis gas for a 2000tons/day methanol plant will now be described.

Example The method of the present invention was applied to thecompression of a methanol synthesis gas which was received from gasreforming at 37 C. and 18.4 kg./Sq. cm., and had the following initialcomposition:

Content in Synthesis Gas Component Mols/hour Mol percent Carbon monoxide3, 08). IL 5 Carbon dioxide. 2, 801 13. 2 Hydrogen, 14, 836 69. 8Methane 4&8 2. 1 Nitrogen.-. 2t) 0. 1 Water vapor 74 0. 3

Following are pertinent operating conditions and compositions ofprincipal process streams.

1 Ambient.

Stream 22 was further compressed to methanol synthesis pressure of 360kg./sq. cm. in two subsequent centrifugal compressors operated inseries, with oil injection prior to the third compressor only, since thegas stream temperature prior to and within the fourth compressorremained below 90 C. The compressor units 4 and 15, and the twosubsequent compressors, were fabricated with carbon steel casings, whichwere not subject to carbon monoxide attack and carbonyl formation, dueto the presence of the injected oil.

We claim:

1. In the method of compression of a gaseous stream containing carbonmonoxide in which said gaseous stream is passed into a rotatingcentrifugal compressor at an initial pressure, said centrifugalcompressor having at least one carbon steel structural element incontact with said gaseous stream, said gaseous stream is compressed insaid centrifugal compressor to a final pressure which is higher than theinitial pressure, and the resulting gaseous stream is discharged fromsaid centrifugal compressor at said final pressure, the improved methodof preventing carbon monoxide reaction with said structural element andcarbonyl formation which comprises injecting a liquid oil into saidgaseous stream at said initial pressure,

said liquid oil having a flash point above the maximum temperature ofsaid gaseous stream within said centrifugal compressor, and separatingsaid liquid oil from the resulting gaseous stream discharged from saidcentrifugal compressor at said final pressure.

2. The method of claim 1, in which said gaseous stream is a methanolsynthesis gas principally containing carbon monoxide, carbon dioxide andhydrogen.

3. The method of claim 2, in which said liquid oil has a flash pointabove 230 C.

4. The method of claim 1, in which said liquid oil is a hydrocarbon oil.

5. The method of claim 1, in which said liquid oil is a mineral oil.

6. The method of claim 1, in which said liquid oil is a silicone oil.

7. The method of claim 1, in which said resulting gaseous streamdischarged from said centrifugal compressor is cooled prior toseparation of said liquid oil.

8. In the method of compression of a gaseous stream containing carbonmonoxide in which said gaseous stream is passed into a first rotatingcentrifugal compressor at an initial pressure, said first compressorhaving at least one carbon steel structural element in contact with saidgaseous stream, said gaseous stream is compressed in saidfirst'compressor to an intermediate pressure which is higher than theinital pressure, the resulting intermediate pressure gaseous stream isdischarged from said first compressor, said intermediate pressuregaseous stream is cooled to a lower temperature, the resulting cooledgaseous stream is passed into a second rotating centrifugal compressorat intermediate pressure, said second compressor having at least onecarbon steel structural element in contact with said gaseous stream,said gaseous stream is compressed in said second compressor to a finalpressure which is higher than the intermediate pressure, and theresulting gaseous stream is discharged from said second compressor atsaid final pressure, the improved method of preventing carbon monoxidereaction with the carbon steel structural elements of said firstcompressor and said second compressor, and thereby preventing carbonylformation, which comprises injecting a liquid oil into said gaseousstream at said initial pressure, said liquid oil having a fiash pontabove the maximum temperature of said gaseous stream within said firstand second centrifugal compressors, and separating liquid oil from theresulting gaseous stream discharged from said second compressor at saidfinal pressure.

9. The method of claim 8, in which said gaseous stream is a methanolsynthesis gas principally containing carbon monoxide, carbon dioxide andhydrogen.

10. The method of claim 9, in which said liquid oil has a flash pointabove 230 C.

11. The method of claim 9, in which a liquid phase containing oil andcondensed liquid water is separated from said cooled intermediatepressure gas stream, and thereafter a second stream of liquid oil isadded to the cooled intermediate pressure gas stream prior to passingsaid gas stream into said second rotating centrifugal compressor.

12. The method of claim 8, in which said liquid oil is a hydrocarbonoil.

13. The method of claim 8, in which said liquid oil is a mineral oil.

14. The method of claim 8, in which said liquid is a silicone oil.

15. The method of claim 8, in which said resulting gaseous streamdischarged from said second compressor at said final pressure is cooledprior to separation of said liquid oil.

16. The method of claim 8, in which additional oil is injected into thecooled intermediate pressure gaseous stream prior to passing saidgaseous stream into said second rotating centrifugal compressor.

No references cited.

ROBERT A. OLEA-RY, Primary Examiner. WILLIAM E. WAYNER, AssistantExaminer.

US. Cl. X.R.

